Polyolefin compositions containing coated mica particles to prevent yellowing and methods therefore

ABSTRACT

Antioxidant containing polyolefin compositions are provided including titanium dioxide-coated mica particles comprising a coating of silica and alumina calcined thereon to prevent yellowing of the compositions. Process for preventing the yellowing of antioxidant containing polyolefin compositions is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to anti-oxidant containing polyolefincompositions including titanium dioxide-coated mica particles with acoating of silica and alumina calcined thereon to prevent yellowing ofthe polyolefin compositions.

2. Description of the Prior Art

Many pearlescent or nacreous pigments are based on micaceous substrateswhich have been coated with a metal oxide layer. As a result ofreflection and refraction of light, these pigments exhibit pearl-likeluster and depending on the thickness of the metal oxide layer, they canalso exhibit interference color effects.

Among the pearlescent pigments encountered most often on a commercialbasis are titanium dioxide-coated mica pearlescent pigments. However,often such pigments are incompatible with the polyolefin compositions inwhich they are incorporated. For instance, inorganic pigments, such astitanium dioxide-coated mica pigments, frequently cause yellowing ofanti-oxidant containing polyolefin compositions in which they arecontained.

U.S. Pat. No. 4,209,430 relates to a process for suppressing yellowingin thermoplastic polyolefins containing a phenolic anti-oxidant and TiO₂pigment by adding a phosphorylated polyene either directly to thepolyolefin or as a coating on the pigment.

U.S. Pat. No. 4,357,170 reviews various solutions to the problem ofyellowing of titanium dioxide pigments in polymeric compositions such aspolyethylene in the presence of additives such as phenolic antioxidants.

U.S. Pat. No. 4,494,993 relates to a pigment in which a homogeneousmixture of titanium dioxide, silicon dioxide and aluminum oxide iscoated on mica flakes.

It has been found that the prior art solutions to the yellowing ofanti-oxidant containing polyolefin compositions are not totallyeffective. Some yellowing of the polyolefin compositions can still beseen. Therefore, it is desirable to develop a titanium dioxide-coatedmica containing polyolefin composition which avoids the disadvantages ofthe prior art described above, affording the advantages of substantiallyimproving control of the yellowing phenomena frequently encountered.

It is accordingly an object of this invention to provide anti-oxidantcontaining polyolefin compositions including titanium dioxide-coatedmica particles having a coating of silica and alumina calcined thereonto prevent yellowing of the polyolefin composition. Another object ofthe present invention is to provide a method of preventing the yellowingof an anti-oxidant containing polyolefin composition.

These and other objects of the invention will become apparent to thoseskilled in this art from the following detailed description.

SUMMARY OF THE INVENTION

This invention relates to anti-oxidant and titanium dioxide-coated micacontaining polyolefin compositions and more particularly to suchpolyolefin compositions in which the titanium dioxide-coated micaceouspearlescent pigments include a coating of silica and alumina calcinedthereon.

The invention also provides a process for preventing the yellowing of apolyolefin composition, which comprises mixing titanium dioxidecoated-mica particles having a coating of silica and alumina calcinedthereon with an antioxidant and a polyolefin.

DETAILED DESCRIPTION OF THE INVENTION

Titanium dioxide-coated mica pearlescent pigments are well known in theart. They are generally formed by depositing a hydrous titanium compoundon a mica substrate and calcining to produce the titanium dioxide-coatedmica pigment. Any of the titanium dioxide-coated mica pigments knownheretofore can be used in the practice of the present invention.

Preferably, a mica substrate is coated with hydrous titanium dioxide.The titanium dioxide-coated substrate may then be calcined.

The titanium dioxide-coated mica pigments include anatase and rutileforms of titanium dioxide. Both forms can be treated with a coating ofsilica and alumina according to the present invention to prevent theyellowing of the polyolefin compositions in which they are contained.Various particle sizes of the titanium dioxide-coated mica pigments maybe coated with the silica and alumina coating. For example, pigmentswith an average particle size from 10μ to 50μ may be coated with thesilica and alumina coating. Particle sizes outside this range can alsobe treated.

The titanium dioxide-coated mica pigments which can be treated with acoating of silica and alumina include the white pearl titaniumdioxide-coated micas. Also, interference pigments can be used.Interference pigments are pigments in which the thickness of thetitanium dioxide layer on the mica causes the film to act as an opticalfilter, and interference colors are observed. Pigments such asinterference yellow, red, blue and green may be used or any intermediatecolors.

The titanium dioxide-coated mica pigments also may contain ingredientsadded thereto to improve the durability characteristics or otherproperties of the pigments. Thus, the titanium dioxide-coated micapigments may contain various oxides such as tin oxide, chromium oxidesand the like.

A wide variety of polyolefins are well known in the art and are suitablefor being pigmented by the coated titanium dioxide-coated mica pigmentsof this invention. Illustrative of such polyolefins are the following:polyethylene, polypropylene, polybutadiene, and the like. The foregoinglist is merely for illustration and not by way of limitation.

Conventional anti-oxidants typically used in polyolefin compositionsinclude, for example, alkylated phenols, substituted benzophenones,thiobisphenols, polyphenols, and the like and mixtures thereof.Exemplary antioxidants include 2,6-di-tertiary-butyl-para-cresol,2,2'-methylenehis (4-methyl-6-tertiary-butylphenol) 4,4'-methylenebis(2,6-di-tertiary-butylphenol), 4,4'-thiobis(6-tertiary-butylorthocresol), and the like.

A wide variety of conventional additives may also be included in thepolyolefin compositions as is necessary, desirable or conventional. Suchadditives include catalysts, initiators, blowing agents, UV stabilizers,organic pigments including tinctorial pigments, plasticizers, levelingagents, flame retardants, anti-cratering additives, and the like.

In practicing the invention, the titanium dioxide-coated mica pigmentmay be treated with a coating of silica and alumina according to themethod disclosed herein. Thus, the titanium dioxide-coated mica pigmentmay be coated or surface treated by contacting a solution of a coatingcomposition with the inorganic pigments followed by drying the pigments.

After the titanium dioxide-coated mica particles are provided with thecoating, it is calcined thereon. The calcining of the titanium dioxidecoating compound on the mica substrate and the calcining of the coatingon the titanium dioxide-coated mica may be done separately in twocalcining steps, or simultaneously in a single calcining step afterapplying a coating composition directly onto the coating of the titaniumcompound. For example, the coating composition for the silica andalumina coating may be added directly to the coating of the titaniumcompound on the mica substrate and then calcined, thereby requiring onlyone calcining step.

In the case of the anatase modification of the titanium dioxide coating,the two calcining steps are preferred. For the rutile modification, boththe single calcining step and the two calcining steps may be used.

Of importance in this treatment, though, is that the final treatedinorganic pigment retain the coating of silica and alumina calcined on asurface thereof. Conventional finishing operations then may be practicedon the treated inorganic pigment.

The coating of the titanium dioxide-coated mica pigment with silica andalumina may be done with the simultaneous coating of a Si compound andan Al compound. However, it is also possible to first coat an Alcompound followed by a Si compound, or a Si compound first followed byan Al compound.

The amount of the coating composition is preferably sufficient toprovide silica and alumina on the titanium dioxide-coated mica in aquantity of about 0.5% to about 2.0% Al by weight, more preferably about0.8% to about 1.5% Al by weight, and about 1.5% to about 4.0% Si byweight, more preferably about 1.5% to about 3.0% Si by weight.

If too great a concentration of silica or alumina are used in thecoating on the titanium dioxide-coated mica, the luster of the pigmentfalls to unacceptable levels. In that event, the concentrations shouldbe reduced.

The range for the calcining temperature of the coating composition forthe silica and alumina coating is preferably about 800° C. to about 950°C. More preferably, the range is about 825° C. to about 875° C. Thecalcining time for the silica and alumina coating can range from severalminutes to several hours. Typically, 100 g of titanium dioxide-coatedmica pigment are calcined at about 850° C. for 45 minutes. If a smallerquantity is used, less calcining time is needed. In general, it is onlynecessary to insure that the pigment reaches the calcining temperaturefor a short period of time.

Sodium silicate, Na₂ SiO₃.5H₂ O, may preferably be used for depositionof the silica. Other forms of soluble silicates can also be used, suchas quaternary ammonium silicate solutions. Aluminum chloride ispreferably used for the precipitation of the alumina. Other compounds ofalumina may also be used such as aluminum nitrate, aluminum acetate oraluminum sulfate. The pH during addition of the coating composition forthe silica and alumina coating is usually about 7.0 to about 9.0. Thepreferable pH is about 7.5 to about 8.5.

The titanium dioxide-coated mica particles having a layer of silica andalumina calcined thereon may then be added to an antioxidant containingpolyolefin composition. The titanium dioxide-coated micaceous productsof the present invention can be used in any application of polyolefincompositions where the titanium dioxide-coated mica pigments have beenused heretofore.

A significant pigmentary property improvement which can be garnered bythe treated titanium dioxide pigments of this invention is suppressionof yellowing in polyolefin compositions, such as, for example,polyethylene, polypropylene and the like.

By way of illustration but without limitation of the scope of theinvention, following are examples of the antioxidant containingpolyolefin compositions having titanium dioxide-coated mica particlesaccording to the present invention.

Example 1

100 g. of fully calcined, pearl reflecting TiO₂ -coated mica withparticles having an average particle size of 10 microns, available fromThe Mearl Corporation as Mearlin Satin White, 9130F, were dispersed in500 ml of distilled water and placed in a suitable vessel. Thetemperature was raised to 85° C. and maintained at this temperature.Stirring was maintained throughout the coating process. A solution wasthen added containing 9.6 g. of AlCl₃.6H₂ O in 150 ml of distilled waterat a rate of 1.5 ml/min. The pH was maintained at 8.0 using a solutionconsisting of 17.0 g. of Na₂ SiO₃.5H₂ O in 150 ml of distilled water.After the addition, the pH was reduced to 3.5 using 1:1 conc. HCl:distilled water. The slurry was stirred for one hour. It was thenfiltered on a Buchner funnel, washed adequately with distilled water toremove salts and dried overnight at 120° C. It was then calcined at 850°C. for 45 minutes. This procedure produced a coating of approximately 2%Si by weight and 1% Al by weight on the TiO₂ -coated mica.

Example 2

The procedure of Example 1 was repeated using a pearl reflecting TiO₂-coated mica having an average particle size of 25 microns and availablefrom The Mearl Corporation as Mearlin Super White, 9020C.

Example 3

The procedure of Example 1 was repeated using a pearl reflecting TiO₂-coated mica having an average particle size of 10 microns and availablefrom The Mearl Corporation as MagnaPearl 2000.

Example 4

In this example, a single calcining step was used after the addition ofthe alumina and silica coating.

Example 1 was repeated, except that fully calcined pearl reflecting TiO₂-coated mica platelets were not used. Instead, Mearlin Satin White 9130Fplatelets were used directly from a coating bath after the hydroustitanium dioxide had been added. The amount of platelets taken wassufficient to yield 100 g. of fully calcined product. The temperature ofthe platelets in the mother liquor was adjusted to 85° C. and theprocedure of Example 1 followed.

The Yellow Index for a control sample of untreated platelets taken fromthe coating bath was 38.1. A sample treated with alumina and silica andcalcined at a temperature of 900° C. has a Yellow Index of 10.0.

Testing was performed by incorporating the silica and alumina coatedtitanium dioxide-coated mica in high density polyethylene (HDPE) stepchips with an antioxidant, butylated hydroxytoluene (BHT), and exposingthe step chips in a QUV meter. The yellowing which occurred wasmonitored by obtaining the Yellow Index as measured on a Hunter LabScanmeter.

The pigment under test was first blended at 1% concentration based onthe weight of the resin with pellets of high density polyethylene (HDPE)in a P-K (Paterson-Kelley) blender. 0.3% butylated hydroxytoluene wasadded and blended. A charge of 200 g was used. The resin containing thetreated titanium dioxide-coated mica and the BHT was then injectionmolded into step chips. These step chips were then exposed in a QUVmeter for a total of 96 hours. The Yellow Index was then measured on aHunter LabScan instrument. The higher the Yellow Index (YI), the greaterthe degree of yellowing. If BHT is not used, no yellowing occurs. Table1 lists the values obtained in using this standard testing procedure onExamples 1, 2 and 3.

                  TABLE 1                                                         ______________________________________                                                        Average                                                                       Particle                                                                      Size (μ)                                                                          YI      Visual Obs.                                    ______________________________________                                        Ex. 1 Pearl TiO.sub.2 Coated Mica -                                                             10       37.4    Yellow                                     Untreated                                                                     Ex. 1 Pearl TiO.sub.2 Coated Mica -                                                             10       8.9     No                                         Treated                            Yellowing                                  Ex. 2 Pearl TiO.sub.2 Coated Mica -                                                             25       43.4    Yellow                                     Untreated                                                                     Ex. 2 Pearl TiO.sub.2 Coated Mica -                                                             25       6.2     No                                         Treated                            Yellowing                                  Ex. 3 Pearl TiO.sub.2 Coated Mica -                                                             10       29.9    Yellow                                     Untreated                                                                     Ex. 3 Pearl TiO.sub.2 Coated Mica -                                                             10       1.5     No                                         Treated                            Yellowing                                  ______________________________________                                    

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art.

What is claimed is:
 1. A composition comprising:a polyolefin; anantioxidant; and mica particles having on a surface thereof a firstlayer of a coating consisting of titanium dioxide and a second layer ofa calcined coating consisting of silica and alumina thereon.
 2. Acomposition according to claim 1, wherein the antioxidant is butylatedhydroxytoluene.
 3. A composition according to claim 1, wherein thepolyolefin is polyethylene.
 4. A composition according to claim 1,wherein the coating of silica and alumina comprises of about 1.5% to4.0% of silicon and of about 0.5% to 2.0% of aluminum, based on theweight of the coated mica.
 5. A composition according to claim 1,wherein the coating of silica and alumina comprises of about 1.5% to3.0% of silicon and of about 0.8% to 1.5% of aluminum, based on theweight of the coated mica.
 6. A composition according to claim 1,wherein the coating of silica and alumina is calcined at a temperatureof about 800° to 950° C.
 7. A composition according to claim 1, whereinthe coating of silica and alumina is calcined at a temperature of about825° to 875° C.
 8. A composition according to claim 1, wherein thecoating of titanium dioxide is the rutile form.
 9. A compositionaccording to claim 1, wherein the coating of titanium dioxide is theanatase form.
 10. An antioxidant containing composition comprising, inaddition to the antioxidant, titanium dioxide-coated mica particles, thetitanium dioxide-coated mica particles having a layer consisting ofsilica and alumina calcined thereon.
 11. A micaceous pigment forpreventing yellowing in antioxidant containing polyolefin compositions,the pigment being mica particles having on a surface thereof a firstlayer of a coating consisting of titanium dioxide and a second layer ofa coating consisting of silica and alumina; the second layer beingcalcined thereon.
 12. A process for preventing the yellowing of anantioxidant containing polyolefin composition, which comprises mixingthe polyolefin with mica particles having on a surface thereof a firstlayer of a coating consisting of titanium dioxide and a second layer ofa coating consisting of silica and alumina, the second layer beingcalcined thereon, and an antioxidant.
 13. The process according to claim12, further comprising tile step of preparing the polyolefin compositionby adding an antioxidant to the polyolefin.
 14. The process according toclaim 12, wherein the antioxidant is butylated hydroxytoluene.
 15. Theprocess according to claim 12, further comprising the step of providingthe second layer of silica and alumina on the first layer of titaniumdioxide.
 16. The process according to claim 12, further comprising thestep of calcining the second layer of silica and alumina.
 17. Theprocess according to claim 12, further comprising the step of providinga layer of titanium dioxide on the mica particles.
 18. The processaccording to claim 12, further comprising the step of preparing apolyolefin composition wherein the polyolefin is polyethylene.
 19. Theprocess according to claim 12, further comprising the step of providinga layer of a coating of silica and alumina.
 20. The process according toclaim 19, wherein the step of providing a layer of silica and aluminacomprises adding the silica before the alumina.
 21. The processaccording to claim 19, wherein the step of providing a layer of silicaand alumina comprises adding the silica after the alumina.
 22. Theprocess according to claim 12, wherein the first layer of a coating oftitanium dioxide is calcined before calcining the second layer of silicaand alumina.
 23. The process according to claim 12, wherein the firstlayer of coating of titanium dioxide is calcined with the second layerof silica and alumina.